Elements of a gas turbine engine are subject to damage when impact of a foreign object takes place during operation of the engine. This may for example be a bird strike or other impact. Fan blades, nose cones (spinners) and compressor aerofoils are particularly susceptible to this type of foreign object damage (FOD). Larger impacts, such as bird strikes, are particularly dangerous and can cause structural damage to the engine. In the design of engines for aircraft, components of a gas turbine engine whose failure is likely to impair the aircraft's ability to continue safe flight are referred to as critical components, and special controls are necessary in the design and operation of such components to guard against their failure. In order to ensure the safe functioning of an engine, it is necessary either to design critical components to be able to resist such impacts, or to detect that impacts have occurred and to take the necessary corrective action before component fatigue causes complete failure of the component.
Designing components to be able to resist significant impact events, and then to have the integrity to continue in safe operation until the next scheduled maintenance, tends to result in their being significantly thicker and heavier than they would otherwise need to be. This has associated disadvantages for the engine as a whole, not least in terms of fuel consumption. However, if components are not to be designed to have such structural resistance, then detection of impact events must be extremely reliable, as such detection will be essential to the safety and integrity of the engine. Without the inherent ability to continue functioning until scheduled maintenance, a damaged component must be detected and corrective action taken at the earliest possible opportunity.
Visual inspection of gas turbine engine blades is hampered by the location of the engine on an aircraft. Fan blades are normally of greatest concern for FOD and while they are visible in most applications, they are also located some distance off the ground, meaning that a detailed inspection is not feasible during normal operational service. Detailed inspection is essential as FOD may not be obvious, or even visible, from a surface inspection. In metallic blades, cupping or minor cracking may occur following a bird strike. Composite blades are likely to show less distortion but may delaminate or suffer other internal damage, which is not easily detectable visually.